1. Field of the Invention
The present invention relates to a radar sensor and a method for operating a radar sensor, e.g., a radar sensor having rapid-chirp radar signals.
2. Description of the Related Art
Modern motor vehicles increasingly have driver assistance systems for supporting a vehicle driver when driving the motor vehicle. For example, such driver assistance systems may inform the vehicle driver about an obstacle while parking. In addition, driver assistance systems may also support the vehicle driver during travel by, for example, detecting an obstacle on the travel route of the motor vehicle and then signaling this potential hazard to the vehicle driver, or possibly even actively intervening into the driving behavior of the motor vehicle.
LFMCW (linear frequency modulated continuous wave) modulation of the radar signals has been used very frequently in the automotive field up to now, due to the low hardware complexity and the low computing complexity. However, with this method, when ascertaining range X and relative velocity v of an object, all targets on a straight line in the X-v space are mapped to a single frequency. This ambiguity may be partially resolved by resolving the spatial points of multiple LFMCW ramps having different ramp slopes in combination with a matching method. One method for determining the range and/or the relative velocity with the aid of an LFMCW radar device is, for example, disclosed in the published German patent application document DE 10 2009 057 191 A1.
Rapid-chirp sequence modulation, for example, provides an alternative to LFMCW modulation. In this modulation type, very short frequency ramps are transmitted in succession. The duration of a frequency ramp is typically in the range from approximately 10 μs up to a few 10 μs. The distance between the individual frequency ramps is only a few microseconds. This rapid-chirp sequence modulation allows a good separation of the targets based solely on different ranges X and relative velocities v due to very short frequency ramps. The range estimation is carried out independently of the relative velocity estimation. Therefore, unlike LFMCW modulation, with rapid-chirp sequence modulation, no ambiguities exist due to overlaps of the range and the relative velocity. For further digital processing, it is necessary to convert the radar signals and the radar echoes received by the radar system into digital signals. For a large unambiguousness range with respect to the range, a high sampling rate is required for the digitization of the radar signals and the received radar echoes. Due to the predefined very short ramp period of the transmitted frequency ramps, the high sampling rate is possible only using a large number of samples. In addition, the unambiguousness and the separability for the detection of objects in radial velocity are a function of the number of frequency ramps and the time difference between consecutive frequency ramps. In short, the system requirements with respect to the unambiguousness and the separability are limited by the requirements on the hardware, for example, the sampling rate, memory capacity, data transmission rate, and computing complexity. In the automotive field, the focus is particularly on low hardware costs and short signal processing times. Therefore, there is a need for an efficient rapid-chirp radar system having both a large unambiguousness range and good separability.